The invention relates to a method primarily adapted for treating a comminuted cellulosic fibrous material with a treatment fluid. In particular, the invention primarily relates to diffusing wash water into a cellulosic pulp mass to displace liquid already in the mass to effect washing thereof.
In early stages of pulp treatment machinery development, diffusion was practiced by providing a plurality of screens located in the bottom of a tall cylindrical tank. Fresh wash water was added by a distribution system to the top of the pulp suspension, and the spent chemical solution was extracted from the tank under the screens being displaced by the fresh water diffusing downwardly by gravity through the pulp mass. Generally such diffusers were operated in series to increase the washing efficiency. Later, stationary pulp diffusers were replaced by rotary vacuum drum filters.
In modern times, the most commercially successful diffusers have operated on a continuous basis. In such modern diffuser designs the pulp mass is pumped upwardly through a tank passing inbetween a series of vertical concentric screens through which the spent liquor is extracted, wash water being introduced by generally tubular nozzles rotating in a concentric path between pairs of concentric screens. Attempts have been made to build such continuous diffusers with stationary extraction screens, however it was found that the screens had a tendency to clog rapidly, resulting in channeling of the pump and resultant low displacement efficiency. In order to overcome such clogging, the screens were made movable, and in commercial installations such screens are reciprocated, moving slowly upwardly at about the speed of pulp flow a given stroke length, and then moving rapidly downwardly (while extraction is shut off) to clean the screens. While such diffusers work very well and have enormous advantages over early stage technology diffuser machinery, the necessity for reciprocating the screens results in undesired complications and expense. Modern day attempts to overcome such disadvantages, such as by providing stationary screens and introducing gas within the screens to prevent clogging, have not met with commercial acceptance.
According to the present invention, a method is provided for the continuous treatment of a fibrous suspension, such as a suspension of comminuted cellulosic fibrous material, which overcomes drawbacks associated with conventional commercially utilized treatment apparatus, while maintaining the advantages thereof. According to the present invention, stationary screens are successfully utilized to effect uniform treatment of the fibrous material; despite the fact that stationary screens are utilized in a continuous treatment of a fibrous suspension, no significant screen clogging--which would result in non-uniform treatment--occurs.
According to one aspect of the present invention, a method of continuously treating a suspension of comminuted cellulosic fibrous material is provided. While the invention is primarily directed to the diffusion washing or bleaching of paper pulp having a consistency of about 6 to 14%, the general principals of the invention are applicable to a variety of other treatment procedures, types of suspensions, and suspension consistencies. For instance the invention is applicable to the thickening of pulp.
An exemplary method of continuously treating a suspension according to the invention utilizes a cylindrical vessel with stationary screens and movable treatment-fluid introduction structures. The method comprises the following steps: (a) Defining a plurality of radial segments, and a plurality of vertical channels in each radial segment, in the vessel. (b) Introducing the suspension, adjacent the vessel bottom upwardly in the vessel in a moving suspension column, into a cross-sectional area in the vessel corresponding to that of approximately one of the radial segments. (c) Introducing treatment liquid with the movable fluid introducing structures so that shock waves acting on the stationary screens as a result of the fluid introduction are minimized. (d) Substantially continuously removing withdrawn fluid from the majority of the stationary screens' area. (e) Terminating extraction from the screens in each radial segment approximately when the suspension is being introduced therein; and (f) Continuously removing from a top portion of the vessel, above the level of the screens, a portion of the entire radial extent of the suspension at the of the top of the column preceding the radial segment into which the suspension is being introduced at the bottom of the column. The further step of (g) controlling the ratio of the upward flow of suspension in each of the vertical channels so that the flow in each channel is substantially the same as the flow in the other channels, is also preferably practiced.
In general, treatment of the suspension, according to the invention, is accomplished by deliberately promoting channeling in a vessel including stationary screen assemblies, and coordinating the suspension introduction, the suspension flow, the suspension removal, the displaced-liquid extraction, and the treatment liquid introduction so that clogging will not occur, and a uniform treatment of the fiber passing through the vessel results.
The preferred apparatus for practicing the method of the present invention includes many novel components, and a synchronization of these components to effect the desired uniform treatment of fiber while maintaining the screens stationary. Preferably, the stationary screens are mounted on a plurality of radially extending extraction arms, the extraction arms defining a plurality of radial segments. The screens preferably include a plurality of annular, concentric, screen assemblies, and the fluid introducing structure comprises a generally annular nozzle assembly disposed between each set of screens, and rotatable with respect to the screens. The screens and nozzle assemblies define a plurality of vertical channels. Upward movement of the suspension is controlled by the segments and channels. While in the ensuing disclosure an exemplary predetermined number of radial segments and vertical channels (i.e. 12 radial segments and 6 vertical channels) will be disclosed, it is to be understood that virtually any number of segments or channels could be provided depending upon the material being treated, the flow rate desired, the diffusion efficiency required, etc. (e.g. 3-48 radial segments and 2-12 vertical channels).
Suspension is introduced into the bottom of the vessel by a rotating pulp inlet structure which has the cross-sectional shape and area of approximately one radial segment. The pulp inlet is connected to a rotating shaft which is centrally located in the vessel, the shaft also being connected to a suspension withdrawal structure, the treatment-liquid introducing nozzle assemblies, and a structure for controlling the ratio of upward flow of suspension in each vertical channel of a radial segment.
The stationary screens are constructed so that they present a minimum resistance to the upward flow of suspension, having the cross-sectional area of a right circular cone frustum, and having the liquid inlet openings in the screen face slanted downwardly. A plurality of screen segments can be stacked upon each other to provide multiple stages, each stacked assembly having the cross-sectional configuration of the frustum of a right circular cone, and including interior passageways connecting each screen of each stage to a different extraction arm. All screen segments within a given radial segment of the vessel are hydraulically connected to the same, single, extraction arm.
The construction of the nozzle assembly according to the present invention is designed to minimize the shock waves acting upon the screens as a result of nozzle movement. The nozzle design--while particularly adapted for use with the stationary-screen apparatus according to the invention--also is applicable to conventional continuous diffusers, such as shown in U.S. Pat. No. 3,524,551. The nozzle assembly includes a vertically extending generally linear nozzle with fluid-introducing openings formed therein, and an annular wall structure concentric with the vessel and annular screens, and having an increasing cross-sectional area from a point immediately following the fluid introducing structure in the direction of rotation of the nozzle to a point of connection of the annular wall structure to the nozzle. The cross-sectional area of the wall structure is such that at any point in the rotation cycle of the nozzle assembly the volume of introduced liquid plus the nozzle volume is a constant. The nozzle may be readily constructed by providing a metal ring as an interior component, and placing a polytetrafluoroethylene ring of continuously varying thickness along each face of the metal ring. The polytetrafluoroethylene exterior surfaces can be formed with circumferential ribs to maintain uniform liquid distribution to minimize resistance.
The suspension withdrawal structure located above the screens comprises a screw conveyor assembly, or like structure, capable of removing an entire vessel radius of material at the same time. This is in contradistinction to prior art removal scrapers, such as shown in U.S. Pat. No. 3,905,766, which move each portion of suspension arcuately as well as radially during each rotation. The screw conveyor assembly includes a rotating screw, which is rotatable about an axis extending along a radius of the vessel, the screw conveyor assembly being rotatable with the central shaft.
Following the screw conveyor assembly in the direction of rotation of the shaft is a means for equalizing the flow rate within the vertical channels of a radial segment or segments over which the conveyor assembly has passed. Such a structure preferably comprises a plate mounted for rotation about a generally horizontal axis, with a force applied thereto a pneumatic or hydraulic cylinder or the like. A plurality of pressure sensors located on the bottom of the plate, one associated with each vertical channel within a radial segment, control the force application provided by the cylinder. The entire assembly is rotatable with the shaft.
Extraction of displaced liquid from each of the extraction arms is controlled by a valve assembly so that extraction continuously takes place from each extraction arm (and screen segments associated therewith) except for the approximate time period during which suspension is being introduced into the radial segment associated with that extraction arm. During at least a portion of the time when the extraction is off, back-washing liquid is introduced into the extraction arm and associated screen segments in order to relieve the pulp from the screen surfaces. A single rotary valve, rotated in synchronization with the central shaft of the vessel, may be provided to effect extraction and back-washing.
By properly positioning the suspension introducing structure, nozzle fluid inlets, suspension withdrawal structure, and channel flow-control structure on the central rotating shaft, and synchronizing the rotation of that shaft with the extraction-back-wash valve, it is possible to efficiently and uniformly treat the fibrous cellulosic material passing past the stationary screens, with minimum potential for screen clogging.
It is the primary object of the present invention to provide a simple and effective method (and component parts thereof) for the continuous treatment of a suspension with a treatment fluid. The invention is particularly applicable to the treatment of pulp suspensions having a consistency of about 6-14%. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.